Laser transmitters have been used in a variety of ways in the construction and surveying industries. In a typical application at a construction site, a laser transmitter projects a plane of laser light for use as a position reference by workers, and by various machines carrying laser receivers. The machines may be operated by workers who view position displays, or may be operated automatically or semi-automatically by control systems that respond to the measured positions and elevations, and to a database of desired positions and elevations.
One type of laser based position and machine control system uses a laser transmitter that projects a thin cylindrical beam of laser light in a horizontal plane or in a precisely tilted plane, and continuously rotates the beam in the plane. A laser receiver, which may include a plurality of photo detectors arranged in a vertical row, is mounted on a mast carried by a construction machine. By sensing the plane of laser light, the machine control system determines the elevation of the receiver, and from that position determines the elevation of various machine elements. A comparison is made between the measured elevations and the desired elevations and the machine is then operated, either automatically or manually, in response to this comparison.
Another type of laser based position and machine control system uses a transmitter that projects a horizontal plane of laser light in all directions simultaneously. To accomplish this, a vertical beam of laser light is projected onto a conical reflector. While providing a very simple construction, a transmitter of this type is somewhat limited in operation and flexibility.
Laser based systems that project a plane of laser light using a rotating beam or a horizontally dispersed plane of light necessarily require that the receiver be relatively precisely positioned to receive the laser light so that position information can be developed. Some systems address this issue by using a transmitter that projects one or more tilted, fan-shaped beams of laser light, rotated about a vertical axis. While fan beam systems provide for a broader range of coverage, such systems may have other draw backs, including a difficulty in extracting complete position information from the receiver signals. Other systems use vertical arrays of photo detectors with more complex receivers, positioned on power extendable masts, to permit detection over a large range of elevations. None of these approaches has proved to be entirely satisfactory because of the difficulty encountered in adjusting mast height to place the receiver in the path of the beam, and the limited amount of vertical movement provided by such a mast.
Accordingly, there is a need for a laser transmitter, receiver, and method of operating the transmitter and receiver, in which the operation of the system is simplified, and in which accurate position and control information is readily available over a large range of elevations.